The cereal aphids Rhopalosiphum padi L. and Sitobion avenae (F.) coexist on the same host plant but occupy different ecological niches. R. padi arrives first in the season and reaches its population peak before S. avenae. In addition, R. padi prefers the stem and lower leaves, whereas S. avenae is found mostly on the ears and upper leaves of the plant. In greenhouse experiments, the likelihood of competition between R. padi and S. avenae on wheat seedlings and tillering plants was evaluated. The reproductive rate of S. avenae on tillering plants was negatively affected by previous infestation by R. padi, and this effect was larger when R. padi infestation started at an earlier plant growth stage. Likewise, previous infestation by R. padi decreased S. avenae reproduction on seedlings. In contrast, previous infestation by S. avenae on the flag leaf of flowering plants did not affect R. padi on that plant part. When both aphid species co-occurred from the beginning, the presence of either aphid species negatively affected the reproductive rate of the other. R. padi consistently had a higher rate of population increase than S. avenae. The presence of the other aphid species did not affect within-plant aphid distribution or alate aphid production. Results were similar on wheat seedlings and tillering plants. Finally, alate S. avenae preferred uninfested seedlings over R. padi-infested ones, whereas no preference was observed when the experiment was performed with tillering plants.